1. Field of the Invention
The present invention relates generally to an air/fuel ratio control system for an internal combustion engine. More particularly, the invention relates to feedback air/fuel ratio control system utilizing an exhaust gas sensor. More particularly still, the present invention relates to switching between feedback control and open loop control for selectively carrying out either one depending upon the engine driving condition.
2. Description of the Prior Art
Recently, there have been developed various air/fuel ratio control systems for decreasing harmful exhaust gas components such as CO, HC, NO.sub.x and so on which may be generated in of an internal combustion engine. The prior art air/fuel ratio control systems, typically control the air/fuel ratio in accordance with the concentration of an exhaust gas component. In such system, a control output is determined according to the sensed exhaust gas component concentration which is used for feedback controlling a fuel supply means such as fuel injector, carburetor or the like.
Generally, such air/fuel ratio control system includes a circuit for determining the difference between the exhaust gas sensor output and reference input which is predetermined corresponding to a target air/fuel ratio of the air-fuel mixture, and a control circuit for generating a control output corresponding to determined difference. By setting the reference input of the difference determining circuit to corresponding to a target air/fuel ratio having a corresponding exhaust gas component concentration which will make the most effectively use an exhaust gas purifier, and by controlling the actual air/fuel ratio to approach to the target ratio, the harmful component in the exhaust gas can be effectively eliminated. For example, where a so-called three-way catalyst is employed as the exhaust gas purifier, the target air/fuel ratio will be approximately the same as a stoichiometric ratio. In such a system, an exhaust gas sensor such as a zirconia oxygen sensor is provided in an exhaust gas passage connecting the combustion chamber to the exhaust gas purifier for measuring an exhaust gas component concentration. Since the output voltage of the exhaust gas sensor varied depending upon its internal resistance, it becomes difficult to accurately measure the exhaust gas component concentration using the exhaust gas sensor in a relatively low temperature condition. Consequently, in the temperature range in which the exhaust gas sensor can not accurately and successfully work, air/fuel control operation should be switched from feedback control to open loop control.
For determining whether the sensor temperature is sufficiently high to accurately determine the exhaust gas component concentration, a given electric current is supplied to the exhaust gas sensor and output voltage of the sensor is measured. A minimum voltage and maximum voltage for defining a range in which the exhaust gas sensor will accurately work, is compared with the sensor output voltage. One means for supplying electric current to the exhaust gas sensor is disclosed in co-pending U.S. patent application Ser. No. 145,987 filed with U.S. Patent and Trademark Office on May 2, 1980 corresponding to Japanese Patent Application No. 54-54061, filed May 4, 1979 by Masaharu ASANO et al.
It is also desirable to vary the reference input of the difference determining circuit according to the output voltage of the exhaust sensor for effectively correcting the sensor output when the sensor temperature is too low to accurately determining an exhaust gas component concentration.
For varying the reference input to the difference determining circuit, a reference input generator is provided with the air/fuel ratio control system. The reference input generator generates a reference signal which is varied according to parameters such as the exhaust gas sensor output voltage which is inputted thereto. On the basis of the reference input and the exhaust gas component concentration, the difference determining circuit determines the difference therebetween. When the sensor output is larger than the reference input, a rich monitoring circuit generates an output signal. In response to output signal fed from the rich monitoring circuit, a switching circuit operates to switch control operation from feedback control to open loop control.
In applicants' system, when the control operation is switched from feedback control to open loop control, the value in the rich monitoring circuit is cleared and the rich monitoring circuit becomes inoperative thereby preventing control operation from frequently being switched between the feedback control and open loop control. However, if the engine is maintaned in an idle condition for a relatively long period, the exhaust gas sensor temperature is lowered thereby increasing its internal impedance. Therefore, the sensor output is increased relative to the exhaust gas component concentration. Therefore, the output of the difference determining circuit will indicate a rich mixture condition even if the air-fuel mixture is actually in a lean condition. As a result, soon after switching the control operation from open loop control to feedback control, feedback control will again be interrupted and open loop control resumed.
If the engine is driven at relatively low engine speeds, it becomes necessary to increase the fuel supply to make the air-fuel mixture richer for eliminating the possibility of engine stall. Such a correcting operation is carried out by open loop control in response to a sensed decreasing of the engine speed lower a given speed. If, by increasing the fuel supply, the engine speed exceeds the given speed, control operation returns to feedback control. However, at this point, the air/fuel ratio of the air-fuel mixture will be normally rich, thereby control operation will again be switched to open loop control. This switching will be cyclically repeated to cause unstable driving of the engine.